Catalytic converters are well known for the removal and/or conversion of the harmful components of exhaust gases. Catalytic converters have a variety of constructions for this purpose. In one form the converter comprises the rigid skeletal monolithic substrate on which there is a catalytic coating. The monolith has a honeycomb-type structure which has a multiplicity of longitudinal channels, typically in parallel, to provide a catalytically coated body having a high surface area.
The rigid, monolithic substrate can be fabricated from ceramics and other materials. Such materials and their construction are described, for example, in U.S. Pat. Nos. 3,331,787 and 3,565,830, each of which is incorporated herein by reference. Alternatively, the monoliths can be fabricated from metal foil.
The monolithic substrate and particularly the multiplicity of channels can be coated with a slurry of a catalytic and/or absorbent material.
One method of coating a prefabricated monolithic substrate is to pump the catalyst slurry into the respective channels and then subject the coated substrate to a drying operation. Such systems have been unsuccessful in providing a uniform coating thickness and a uniform coating profile wherein the catalyst coating is deposited over the same length of each of the channels.
It has been proposed to employ a vacuum to draw the catalyst slurry upwardly through the channels. For example, Peter D. Young, U.S. Pat. No. 4,384,014 discloses the creation of a vacuum over the monolithic substrate to remove air from the channels and then drawing the catalyst slurry upwardly through the channels. The vacuum is then broken and excess slurry is removed, preferably by gravity drainage.
James R. Reed, et al., U.S. Pat. No. 4,191,126, discloses the dipping of the monolithic substrate into a slurry and then utilizing subatmospheric pressure to purge the excess coating slurry from the surfaces of the support. The applied vacuum is intended to unplug the channels so that the slurry is drawn over the surfaces of each of the channels.
An improvement in these systems is disclosed in Thomas Shimrock, et al., U.S. Pat. No. 4,609,563. This system encompasses a method of vacuum coating ceramic substrate members with a slurry of refractory and/or catalyst metal components wherein precisely controlled, predetermined amounts of the slurry are metered for application to the ceramic monolithic substrate. The monolithic substrate is lowered into a vessel, also known as a dip pan, of preferably predetermined dimensions to a predetermined depth containing the precise amount of slurry which is to be coated onto the substrate. The slurry is then drawn up by a vacuum which is applied to the end of the substrate opposite to the end which is immersed in the bath. No draining or purging of excess coating slurry from the substrate is necessary nor is any pre-vacuum application step required to eliminate air.
A further improved method is disclosed Victor Rosynsky et al. in U.S. Pat. No. 5,866,210 entitled, “METHOD FOR COATING A SUBSTRATE.” There is disclosed a vacuum infusion method for coating monolithic substrates in which each of the channels comprising the substrate is coated with the same thickness of the coating and is characterized by a uniform coating profile wherein each channel of the substrate is coated over the same length. In particular, the method is directed to a vacuum infusion method for coating a substrate having a plurality of channels with a coating media comprising:                a) partially immersing the substrate into a vessel containing a bath of the coating media, said vessel containing an amount of coating media sufficient to coat the substrate to a desired level without reducing the level of the coating media within the vessel to below the level of the immersed substrate;        b) applying a vacuum to the partially immersed substrate at an intensity and a time sufficient to draw the coating media upwardly from the bath into each of the channels to form a uniform coating profile therein; and        c) removing the substrate from the bath.        
Optionally, after the coating media is applied to the substrate and as the substrate is being removed from the bath, a vacuum continues to be applied to the substrate at an intensity equal to or greater than the intensity of the vacuum imposed on the partially immersed substrate. After the vacuum is imposed, the substrate is inverted and coated from an opposite end producing two coatings having uniform coating profiles. This procedure is known to reduce the overlap of the coating media.
Overlap of the coating compound is particularly problematic to the automobile industry. The overlap area increases the delta pressure across the catalyst converter which adversely impacts engine performance and fuel consumption and increases the engine's wear. Equally problematic is where the coating compound has a gap in the middle which results from incomplete coverage during the coating procedure. This, too, may adversely affect converter and engine performance. However, where the substrate is inverted in order to coat both ends, it is extremely difficult to ensure that the coating materials will not overlap or gap to some extent. Even the smallest amount of overlap or gap negatively affects catalytic performance.
It would therefore be a significant benefit in the art of coating monolithic substrates and particularly monolithic substrates for use in catalytic converters if each channel can be coated with the same thickness of coating for the same length without any overlap or gap in the coating materials.